1. Field of the Invention
The present invention relates generally to control valves, and more particularly, it relates to a mechanism specially adapted to adjust the set point of a pressure differential control valve found in the fuel line to furnaces, boilers and the like.
2. Description of the Prior Art
It is known that the ratio of air to fuel in the mixture fed to burners for furnaces, boilers and the like must be carefully controlled for efficient burning. Too much air wastes energy by heating the unused air. Too little air leads to incomplete combustion of the fuel, which in addition to wasting energy, forms pollutants and potentially leads to explosive conditions when the unburned fuel is again exposed to air.
Typically, the total amount of fuel and air fed to a burner is primarily controlled on load. That is, a load controller increases (or decreases) the flow of both fuel and air to the burners when it senses that the load has, or is about to, increase (or decrease). The fuel flow is varied by a fuel control valve (FCV) in the fuel line and the air flow is adjusted by an air damper in the windbox. In the simplest form of such control, the air and the fuel are continuously adjusted in a fixed ratio which is chosen as optimum at start-up in view of the fuel characteristics and ambient conditions prevailing at that time. As the fuel characteristics and the ambient conditions change, however, the optimum air to fuel ratio changes and the simple load control scheme is unable to respond.
To maintain an optimum air to fuel balance over time, it is common to marginally adjust the air flow or the fuel flow, or both, in response to one or more measured characteristics of the stack gases, typically oxygen content. One approach marginally adjusts the air flow by trimming the position of the air damper which is under the primary control of the load controller. Typically this is accomplished by placing an adjustable mechanical linkage between the damper actuator and the damper which imposes a trim adjustment on the damper in response to an oxygen controller, or the like, adapted to maintain the optimum combustion characteristics in the burners. While this approach generally works, it suffers from certain disadvantages. Failure of the mechanical linkage can, in the worst case, stop the flow of air entirely which leads to a highly explosive condition. Moreover, the percentage adjustment (trim) imposed at a given load condition will not necessarily correspond to the percentage change required when the damper position has been changed by the load controller. Such nonlinear response of the trim controller in respect to the primary controller can lead to instability in the trim control system, particularly during periods when the load fluctuates rapidly.
To compensate for variations in pressure in the incoming fuel, it is known to place a differential pressure control valve (DPCV) in the fuel line ahead of the FCV. The DPCV used is typically a self-operated pressure control valve having a diaphragm actuator actuated by the fuel line pressure. By connecting one diaphragm tap downstream of the FCV and the other diaphragm tap upstream of the FCV, the DPCV responds to variations in the pressure differential across the FCV to maintain that differential pressure at a constant set point. An increase in the differential pressure across the FCV acts to close the DPCV, slowing the flow of fuel and reducing the differential pressure across the FCV. Conversely, a decrease in the differential pressure across the FCV acts to open the DPCV and to increase the differential pressure across the FCV.
Heretofore, it has been known to provide such DPCV with means for manually adjusting the set point only. The DPCV would be adjusted manually by the user at start-up of the system and thereafter would act to control the differential pressure across the FCV at the preselected fixed value.